1. Field of the Invention
The present invention relates to recording and reading information on a flexible record device such as a flexible video disc and, more particularly, to methods and means for reducing undesirable effects of dirt particles on such recording and reading.
2. Description Relative to the Prior Art
In order to record video information at a relatively high packing density on a flexible video disc, the video information is commonly recorded in the form of micron and sub-micron sized data bits such as pits, bumps, apertures, etc. Focusing a recording beam to such small dimensions calls for the use of a high numerical aperture optical system. Attendant with the use of a high numerical aperture optical system is an extremely small depth of focus. As used herein, the phrase "depth of focus" is intended to mean the range of distances along the optical axis of the recording beam optical system within which the recording beam is in acceptable focus, i.e., is capable of producing acceptable quality recording on the flexible record device. As an example, if the recorded video information is in the form of data bits as small as half a micron in size, the maximum allowable depth of focus can be as small as one-fourth of a micron, depending upon the wavelength of the recording beam. Keeping the record surface within this depth of focus is difficult even in a static system, i.e., one in which the optical system and flexible record device are not moving. The difficulty, as might be expected, is multiplied in a dynamic recording system wherein the record device and/or the optical system are in motion.
In a conventional video disc recording system, a 12-inch disc spins at a high rate of speed, commonly 1800 rpm. During recording, a modulated recording bean scans the disc so that many video data bits are recorded along a spiral track. All during this recording process the recording beam must be kept in proper focus, which means that the spinning disc surface must be kept within the depth of focus of the recording beam. As stated, the depth of focus for such a system is extremely small and it is very difficult to maintain proper focus.
Disc recording systems which include means for adjusting or maintaining proper focus have been disclosed in the prior art. See, for example, U.S. Pat. Nos. 3,825,323 of Landwer issued July 23, 1974, 4,033,059 of Sugiura et al issued Jan. 11, 1977, and 4,006,294 of Canino issued Feb. 1, 1977. Such systems produce and sense a degree of capacitance or air pressure which is proportional to the distance between a recording beam lens system and a video disc resting on a rigid platen. When a change in capacitance or air pressure is sensed, a servomechanism responds by attempting to reposition the lens system to keep the recording beam in focus on the disc. When a small, localized disc surface irregularity is encountered by the system, even if it is detected as a change in capacitance or air pressure, the lens positioning adjustment may have to take place so fast that the required compensation is beyond the response time of the servomechanism, especially if the lens system is relatively heavy. Therefore, such prior-art recording systems mainly depend upon the video disc to provide a flat surface, preferably an optically flat surface, against which to float or locate in order to define a focal plane. While the air pressure-sensitive recording system of U.S. Pat. No. 4,006,294 is disclosed for use with a flexible video disc lying on top of a rigid turntable, it is doubtful that such an arrangement would perform satisfactorily in those applications wherein it is required to record submicron-sized data bits, where the depth of focus can be as small as a quarter micron.
In particular, if dirt particles have become imbedded in the surface of the flexible disc which contacts the rigid platen or turntable, or if dirt particles have been trapped between the disc and the platen, or if the platen itself has bumps on it, the disc will not lie flat on the platen. Rather, the dirt particles or bumps will cause a "tent-pole" effect; i.e., in the area of each dirt particle the recording surface (i.e., the surface of the disc facing away from the platen) will be pushed up, possibly out of the depth of focus of the recording beam, thus necessitating impossibly rapid focus adjustments of the lens system. The lens system may even literally vibrate in an effort to maintain proper focus. Accurate recording cannot take place under such conditions.
A number of patents discuss the use of a dynamic air cushion between the flexible disc and the rigid platen to avoid such problems with bumps or particles of dirt and dust. See, for example, U.S. Pat. Nos. 4,060,248 of Lakerveld et al issued Nov. 29, 1977, and 3,947,888 of Jarsen issued Mar. 30, 1976. While such dynamic air cushion systems may reduce the effect of dirt particles, they add significant expense and complexity to an apparatus for recording or reading a flexible video disc, since air direction, pressure and consistency must be very precisely designed and maintained.
In another context, Canadian Pat. No. 698,162 of Damerau et al issued Nov. 17, 1964, suggests using a static air cushion trapped between a resilient layer and a flexible magnetic recording disc to compensate for irregularities in a rigid platen. However, in such a system the disc can sag into the air cushion, creating hills and valleys. In fact, in the magnetic recording system addressed by Canadian Pat. No. 698,162, it is desired that the disc not remain flat because the recording method for such a disc consists of a magnetic recording head resting physically upon the disc and creating a valley at the point of contact. At any rate, such a system would not be able to achieve the high packing densities of information necessary for practical recording of video information, and it is nowhere suggested that such a system could be used for practical video recording or that the disc described in this Canadian patent could be adapted to be recorded upon by a focused energy recording beam.
It is, therefore, apparent that a need exists for methods and means for reducing the effect of dirt particles sandwiched between a flexible record device and a rigid platen and, preferably, a means which does not entail the expense and complexity associated with the dynamic air cushion systems discussed above. The present invention provides such methods and means.
It should also be noted that different problems can arise when dirt particles are imbedded in or rest upon the recording surface of a video disc (that is, the surface of the disc facing away from the rigid platen). However, it is not the purpose of the present invention to address or provide a solution for such problems.